1. Field of the Invention
The present invention relates to a battery stack that is configured of a plurality of battery packs. In the battery stack in which the plurality, of battery packs is stacked, the individual battery pack is sometimes referred to as a “battery cell” or simply as a “cell”.
2. Description of Related Art
There is a case where a high-voltage or large-capacity battery is realized by connecting a plurality of battery packs in series or in parallel. Such a battery is referred to as a battery stack. The individual battery pack tends to have a flat shape. Japanese Patent Application Publication No. 2003-242956 (JP 2003-242956 A) discloses a battery stack in which a plurality of flat battery packs is aligned and connected in series.
For the battery pack, a representative example of which is a lithium-ion battery, an electrolyte and a battery main body are housed in a sealed flat case. Since the case is sealed, an internal pressure thereof is increased, and the case is inflated in a case of a temperature increase or in a case where the case is brought to a high elevation place where the atmospheric pressure is low. In addition, the battery pack generates heat when being charged. Thus, every time the battery pack is charged, the case is inflated.
A technique of preventing inflation of the case that is caused by an increase in the internal pressure is disclosed in Japanese Patent Application Publication No. 2006-040879 (JP 2006-040879 A), for example. The technique is to provide a trapezoidal recess or a trapezoidal projection at the center of a flat surface of a flat case, so as to enhance strength of the case. Here, the flat surface of the flat case corresponds to a lateral surface with the largest area of the case and is also referred to as a wide surface.